This invention relates to printing apparatus, and especially to roller-type printing devices employing type segments formed of resilient material such as rubber for applying various indicia to flat, irregular or uneven surfaces. More particularly, the invention relates to an improved type segment comprising an elongated, flexible strip with a print face molded therein and adapted to be maintained on a curved, generally cylindrical surface, such as on a printing roll.
In the manufacture of packaged goods, certain marking, such as date codes, names, coded identifiers, and the like, are printed on the packages as they are being conveyed at a high rate of speed. Such packaging may be of metal, plastic, or cardboard, for example, and in many instances the marking must be printed on uneven and irregular surfaces.
One type of package and product marking that has just recently come into widespread use is the bar code, which comprises a series of parallel lines or bars of varying width and spacing, containing coded information that can be read by an optical device. These codes must be printed very accurately and without variations in the predetermined incremental dimensions. Obviously, the printing device must be capable of accurate marking regardless of variations in the shape, etc. of the surface to be marked.
The type segments conventionally used are formed or molded of relatively soft elastic material, such as rubber, such that the force of the type imprint can determine the clarity of the marking. If too forceful, the type distorts and produces an unclear print or smudge. If not forceful enough, the marking is not clear enough to be read, and in the case of bar codes is not of sufficient clarity to be processed by an optical reader.
Elongated type segments of elastomeric material usually carry a sequence of letters or numbers seriatim or a bar code of the type described. If the surface to be marked is irregular, the segment must essentially conform to such irregularities, or else some portion or portions of the surface will not be imprinted while others will be smeared.
Various printing wheel devices and elongated elastomeric strips forming type segments have been employed to make clear ink imprints at high speeds, certain of these being the subject of U.S. Pat. Nos. 3,071,071; 3,093,070; 3,230,880; 3,327,624; 3,968,747; and 4,129,074.
Such devices in various respects rely on the yieldability of the type segment to secure a thoroughly uniform imprinting force of all the type elements on an irregular surface being marked. However, if the type segment is not sufficiently yieldable at a high point on the surface, undue force can occur, causing a distorted or smeared mark.
The problems described above are particularly applicable in printing boxboard cartons. In addition to the variations in positioning of the printing roller and the surface of the carton to be marked, other variations occur as a result of the different amounts of support that the carton and its contents provide for the surface. For any combination of ink, type, and carton surface to be marked, there is a range of unit pressure in the zone of contact that works best. Accordingly, the type segment must be designed to compensate for a wide range of carton-surface error while maintaining a nearly uniform type face printing pressure. With such nearly uniform pressure, distortion of the type face will be minimized.
Prior art attempts to meet the requirements described above have included: (1) rubber type that is obviously more resilient than metal type and permits the use of greater marking force; (2) rubber type with a relief in the base of the type holder that permits the type segment to flex as a simply supported beam (e.g., U.S. Pat. No. 3,071,071); and (3) the use of low durometer elastomeric material with a deep section.
All of these prior art techniques, however, have certain deficiencies. Because rubber-type segments, for economic reasons, must be molded flat, the segment will be distorted when flexed to comply with the cylindrical surface of the printing roll. With reference to beam theory, when the segment is flex within its proportional limits, there are surface strains, both compressive and tensile, as a function of the distance between the neutral axis of the section and the radius of curvature. For any given roll diameter, the deeper the type, the more the distortion of the type face.
While Poisson's ratio will cause some distortion in the axial direction, the principal distortion will be in the circumferential direction.
The spring rate of a simple bar in tension is represented by the equation: ##EQU1## where A is the area under load, L is the length of the section under load, and E is the modulus of elasticity (Young's modulus) of the material. From this equation, it follows that for optimum performance, the type segment should be as deep as possible (i.e., should have a large L) for a given type face area, and the neutral axis of the type segment should be as close to the printing face as possible. This would reduce the distortion at the type face at the expense of distortion at the base or support side, which would not be objectionable.
The type segment construction of the present invention satisfies the requirement described above, and affords other features and advantages heretofore not obtainable.